An optical pickup is installed in information apparatus such as a player, a recorder and a drive for leading out or recording information from or to an optical information recording medium, referred also to as simply a medium, such as an MO, CD and DVD. The optical pickup apparatus has an optical element such as a lens for irradiating light generated from a light source having a prescribed wavelength to the medium, and for receiving the reflected light by a light receiving element unit, and a lens for condensing the light on the reflective surface of the medium or the light receiving element.
A plastic is preferably applied for the material of the optical element of the optical pickup apparatus because the optical element can be produced with low cost by a means such as an injection molding. A copolymer of a cyclic olefin and an α-olefin is known as the plastic capable for applying the optical element (for example, refer to Patent Document 1).
In an information apparatus capable of leading and writing information to plural kinds of recording media such as a CD/DVD player, it is necessary that the optical pickup has a constitution capable of corresponding to the wavelength of the light to be applied to each of the media and the shape thereof. In such the case, the optical element is preferably one commonly applicable to both of the media from the viewpoint of cost and picking up property.
In the optical element unit using plastic as a material, it is desirable that the plastic is a material having optical stability similar to that of a glass lens. The optical plastic material such as the cyclic olefin, for example, has sufficiently improved stability in the refractive index with respect to humidity, but the improvement in the stability of the refractive index is not fully sufficient in the present stage.
Various methods have been proposed for compensating the refractive index of such the plastic lens by using a particle filler.
The particle filler is employed for compensating the refractive index of the optical plastic. The plastic can hold sufficient transparency for the lens without light scattering caused by the filler by the use of the filler of sufficiently small particle size. Technology for enhancing the refractive index of the plastic by the addition of the particles is described in Non-patent Documents 1 and 2, for example.
The transparent plastic materials have been employed for various uses in the optical systems since those materials are lighter in the weight and lower in the cost when compared with glass. However, the plastic is inferior to the glass in the stability in the refractive index against changes in temperature and humidity. Therefore, improvement is desired in this point of view for the plastic materials.
The refractive index of an organic polymer decreases with increasing temperature almost without exception (temperature dependency of the refractive index: dn/dT<0). As described below, for example, dn/dT of organic polymer materials (thermoplastic resins) to be applied for the optical use is generally almost −10−4/K.
A method for reducing the absolute value of dn/dT has been proposed, in which a substance having a dn/dT of more than 0 is mixed in the thermoplastic resin host material which has a dn/dT of less than 0. Among inorganic substances, known are the ones showing a dn/dT of more than 0, which is a result of the variation in the intramolecular coordination with varing temperature. It has been expected that the absolute value of dn/dT is reduced by mixing an organic polymer thermoplastic resin of which a dn/dT is less than 0 with inorganic particles having a dn/dT of more than 0. Accordingly, an optical product reduced in thermal sensitivity of an optical property, containing a thermal sensitive thermoplastic polymer resin and particles dispersed therein has been proposed in Patent Documents 2 through 8, for example.
Various proposals have been submitted regarding the composite materials each containing inorganic particles. Composite materials containing particles of a semi-conductive substance and a resin composition in which semi-conductive particles are bonded with the polymer chain by a covalent bond are disclosed, for example, in Patent Document 9, and a resin composition containing zinc sulfide particles is disclosed, for example, in Patent Document 10.
Patent Document 6, for example, describes that the mixing of 40% by weight or more of aluminum oxide or magnesium oxide is necessary to reduce by 50% of the dn/dT of the thermoplastic resin.
However, when a composite material contains inorganic particles having high refractive index in such the high ratio, the following problems may occur, for example, (i) transparency of the composite material largely decreases; and (ii) the property of the composite material varies during prolonged storage due to coagulation of the inorganic particles dispersed in the resin. Accordingly, composite materials suitable for practical use as an optical element have hardly been obtained.
In the methods disclosed in Patent Documents 9 and 10, particles having high refractive index, namely semi-conductive particles are added to increase the refractive index of the resin composition. However, a sufficiently high transparency as an optical element cannot be obtained in the resin composition obtained by such a method. These Patent Documents do not describe the use of two or more kinds of particles having different refractive indexes such as that described in the present invention, and no composite material having high transparency and small temperature dependency in the refractive index as obtained in the present invention has been known until now.
Non-patent Document 1: C. Becker, P. Mueller and H. Schmidt, “Optical and thermomechanical investigations on thermoplastic nanocomposites with surface-modified silica nanoparticles”, SPIE Proceedings, July 1998, vol. 3469, p.p. 88-98
Non-patent Document 2: B. Braune, P. Mueller and H. Schmidt, “Tantalum oxide nanomers for optical applications”, SPIE Proceedings, July 1998, vol. 3469, p.p. 124-132
Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2002-105131, (p. 4)
Patent Document 2: JP-A No. 2002-207101, (Claims)
Patent Document 3: JP-A No. 2003-240901, (Claims)
Patent Document 4: JP-A No. 2002-241560, (Claims)
Patent Document 5: JP-A No. 2002-241569, (Claims)
Patent Document 6: JP-A No. 2002-241592, (Claims)
Patent Document 7: JP-A No. 2002-241612, (Claims)
Patent Document 8: JP-A No. 2002-303701, (Claims)
Patent Document 9: JP-A No. 2002-105325, (Claims)
Patent Document 10: JP-A No. 2003-73563, (Claims)